A conditional switched rate filter system having a two stage digital filter to remove noise from the input signal. The first stage of the filter effectively removes the random noise from the input signal higher in frequency (cut-off frequency) than outside the characteristic response of the closed loop system. The second stage of the filter is used to filter out noise that is at same frequency response as the closed loop system.
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1. A conditional switched rate low pass filter for reducing noise in a closed-loop control system comprising: a first low pass digital filter, the input of which is coupled to an output of the control system, wherein the first digital filter removes noise, in the signal from the output of the control system, higher in frequency than the characteristic response of the control system; a second low pass digital filter, the input of which is coupled to the output of the first digital filter, and the output of which is coupled back into the control system; wherein the second digital filter removes noise which is at essentially the same frequency as the closed-loop system response; buffer memory for storing an output signal from the first digital filter; a signal stability monitor module, coupled to the buffer memory, configured to detect changes in the output signal from the first digital filter; and a computer implementing the first digital filter, the second digital filter, and the signal stability monitor module, coupled to the buffer memory, for controlling the operation of the switched rate low pass filter; wherein the signal stability monitor determines the response time of the second digital filter based on monitoring of the output signal from the first digital filter, as stored in the buffer memory, and wherein: if the output signal has changed by more than a predetermined value within a predetermined time period, the cutoff frequency of the second digital filter is increased to a predetermined value; and if the output signal maxima and minima have been stable within a predetermined range for at least a predetermined period of time, the cutoff frequency of the second digital filter is decreased to a predetermined value.
A noise reduction system for a closed-loop control system uses a conditional switched-rate low-pass filter. It includes a first digital filter that removes high-frequency noise from the control system's output, specifically noise above the control system's characteristic response. A second digital filter further reduces noise at the same frequency as the closed-loop system response. A buffer stores the first filter's output. A stability monitor detects changes in this stored signal. A computer implements both filters and the monitor, controlling the filter's operation. The monitor adjusts the second filter's cutoff frequency based on the first filter's output: if the signal changes rapidly (more than a set amount within a set time), the second filter's cutoff frequency increases; if the signal is stable (within a set range for a set time), the cutoff frequency decreases.
2. The system of claim 1 , wherein the cutoff frequency of the second filter is changed in one step.
The system described previously that reduces noise in a closed-loop control system via conditional switched-rate filtering adjusts the cutoff frequency of the second noise-reducing digital filter in a single, discrete step rather than gradually. This means the frequency changes from one value to another instantaneously, based on the output signal stability from the first filter.
3. The system of claim 1 , wherein the cutoff frequency of the second filter is changed gradually over a period of time.
The system described previously that reduces noise in a closed-loop control system via conditional switched-rate filtering adjusts the cutoff frequency of the second noise-reducing digital filter gradually over a period of time rather than in a single step. This means the frequency changes smoothly, increasing or decreasing to the desired value over a defined duration, based on the output signal stability from the first filter.
4. The system of claim 1 , wherein criteria for increasing the frequency response of the second filter are different than criteria for decreasing the frequency response thereof.
The system described previously that reduces noise in a closed-loop control system via conditional switched-rate filtering uses different criteria for increasing the cutoff frequency of the second filter versus decreasing it. For example, the threshold for signal change that triggers an increase might be different than the stability threshold that triggers a decrease. This allows for asymmetric response to noise fluctuations.
5. The system of claim 1 , wherein: the second low pass digital filter can be set to either a fast response time mode or to a slow response time mode, and is initially set to the slow response time mode; and if the second low pass digital filter is set to the slow response time mode, then if the output signal from the first digital filter has changed by more than the predetermined value within the predetermined time period, the cutoff frequency of the second digital filter is increased to the predetermined value, and the signal stability monitor is configured to check for stability of the output signal; if the second low pass digital filter is set to the fast response time mode, then if the output signal maxima and minima have been within the predetermined range for at least a predetermined period of time the, cutoff frequency of the second digital filter is decreased to the predetermined value, and the signal stability monitor is configured to check for large changes in the output signal.
The noise reduction system described previously that uses conditional switched-rate filtering can set the second digital filter to either a "fast response" or a "slow response" mode, initially starting in "slow" mode. If the second filter is in "slow" mode, and the first filter's output changes rapidly, the second filter's cutoff frequency increases, and the system checks for signal stability. Conversely, if the second filter is in "fast" mode, and the first filter's output has been stable for a while, the second filter's cutoff frequency decreases, and the system monitors for sudden signal changes.
6. The system of claim 5 , wherein the cutoff frequency of the second filter is changed in one step.
The system described previously that can set the second digital filter to either a "fast response" or a "slow response" mode adjusts the cutoff frequency of the second noise-reducing digital filter in a single, discrete step rather than gradually. This means the frequency changes from one value to another instantaneously based on the output signal stability from the first filter.
7. The system of claim 5 , wherein the cutoff frequency of the second filter is changed gradually over a period of time.
The system described previously that can set the second digital filter to either a "fast response" or a "slow response" mode adjusts the cutoff frequency of the second noise-reducing digital filter gradually over a period of time rather than in a single step. This means the frequency changes smoothly, increasing or decreasing to the desired value over a defined duration.
8. A method for conditionally switching the cutoff frequency of a digital filter to reduce noise in a closed-loop control system comprising: filtering an output of the control system via a first low pass digital filter stage; receiving the output signal from the first low pass filter stage via a second low pass digital filter stage; monitoring the output signal from the first low pass digital filter stage to determine the stability of the output signal; wherein: if the output signal has changed by more than a predetermined value within a predetermined time period, the cutoff frequency of the second low pass digital filter stage is increased to a predetermined value; and if the output signal maxima and minima have been stable within a predetermined range for a predetermined period of time, the cutoff frequency of the second low pass digital filter stage is decreased to a predetermined value.
A method for reducing noise in a closed-loop control system involves conditionally switching the cutoff frequency of a digital filter. The method filters the control system's output using a first low-pass digital filter. The output signal is then received by a second low-pass digital filter. The output signal from the first filter is monitored to determine signal stability. If the signal changes rapidly (more than a set amount within a set time), the second filter's cutoff frequency increases. If the signal is stable (within a set range for a set time), the cutoff frequency decreases.
9. The method of claim 8 , wherein: the second low pass digital filter stage can be set to either a fast response time mode or to a slow response time mode, and is initially set to the slow response time mode; and if the second low pass digital filter stage is set to the slow response time mode, then if the output signal from the first digital filter stage has changed by more than the predetermined value within the predetermined time period, the cutoff frequency of the second digital filter stage is increased to the predetermined value, and the signal stability monitor is configured to check for stability of the output signal; if the second low pass digital filter stage is set to the fast response time mode, then if the output signal maxima and minima have been within the predetermined range for at least a predetermined period of time the, cutoff frequency of the second digital filter stage is decreased to the predetermined value, and the signal stability monitor is configured to check for large changes in the output signal.
The method described previously for conditionally switching the cutoff frequency of a digital filter can set the second digital filter to either a "fast response" or a "slow response" mode, initially starting in "slow" mode. If the second filter is in "slow" mode, and the first filter's output changes rapidly, the second filter's cutoff frequency increases, and the system checks for signal stability. Conversely, if the second filter is in "fast" mode, and the first filter's output has been stable for a while, the second filter's cutoff frequency decreases, and the system monitors for sudden signal changes.
10. The system of claim 8 , wherein the cutoff frequency of the second filter stage is changed in one step.
The method described previously that reduces noise in a closed-loop control system via conditional switched-rate filtering adjusts the cutoff frequency of the second noise-reducing digital filter in a single, discrete step rather than gradually. This means the frequency changes from one value to another instantaneously, based on the output signal stability from the first filter.
11. The system of claim 8 , wherein the cutoff frequency of the second filter stage is changed gradually over a period of time.
The method described previously that reduces noise in a closed-loop control system via conditional switched-rate filtering adjusts the cutoff frequency of the second noise-reducing digital filter gradually over a period of time rather than in a single step. This means the frequency changes smoothly, increasing or decreasing to the desired value over a defined duration, based on the output signal stability from the first filter.
12. The system of claim 8 , wherein criteria for increasing the frequency response of the second filter stage are different than criteria for decreasing the frequency response thereof.
The method described previously that reduces noise in a closed-loop control system via conditional switched-rate filtering uses different criteria for increasing the cutoff frequency of the second filter versus decreasing it. For example, the threshold for signal change that triggers an increase might be different than the stability threshold that triggers a decrease. This allows for asymmetric response to noise fluctuations.
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September 5, 2014
June 20, 2017
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